Hair grows in a cycle. That is a scientific reality. There are three phases in the human hair growth cycle and each one of them is a distinct length of time . The anagen phase, or the growing phase, is when the follicle is active and hair grows. This phase lasts for an average of 3 years (range, 2-6 years or more) . The catagen phase, or transitional phase, is when the hair essentially stops growing as the follicle is preparing to renew itself in preparation for the new hair fiber, and lasts for about 2-3 weeks . The follicle shrinks in size and holds the current hair fiber in place. The telogen phase, or resting phase, which lasts for about 3 months, is when the follicle goes dormant; towards the end of this phase, when the follicle begins to reactivate, the hair will shed . Within two weeks, a new hair fiber will begin to grow in its place thus continuing the cycle.
These phases happen concurrently meaning all the phases are happening at the same time in your scalp, and also randomly. About 85%-90% of all the follicles in your scalp will be in the anagen (growing) phase at any given time, 2%-3% of all the follicles will be in the catagen (transitional) phase at any given time, and about 10%-15% of all the follicles will be in the telogen (dormant) phase at any given time . The percentage of hair in each phase varies between person to person and can be affected by factors such as hormones, nutrition, and aging. For example, hormonal changes from pregnancy can induce the percentage of hair follicles in the anagen phase to increase but return back to the telogen phase after 3-6 months postpartum .
So based on this information, do terminal hair lengths exist?
Yes and no. I believe part of the confusion is just in the terminology used to describe the phenomenon. Technically, terminal length does not exist. The maximum length of your hair will always vary based on internal and external factors that can affect hair growth and retention. A better term to use is “terminal time”. Each hair follicle in your head exists for a maximum length of time before it renews itself. This appears to be predetermined by genetics. So basically, we have X amount of time to grow our hair before it sheds and starts all over again. Some people can grow more hair in the same amount of time as others, resulting in longer hair. Some people have longer anagen (growing) phases than others giving more time for hair to grow, resulting in longer hair (that’s if growth rate is the same between the two groups). The combination of the anagen phase (“terminal time”) and the growth rate determines maximum hair length. They are both factors that appear to function independent of each other.
I personally think the terminal hair length discussion is better served by asking these two sets of questions “is there a terminal length of time in which follicles stay in the anagen phase?” and “is there a terminal rate at which individual hair fibers grow?”